[sdiy] Transistor Sallen-Key lowpass filter question

Tristan tu at alphalink.com.au
Mon Nov 19 01:39:02 CET 2018

Is V1 a pure AC source? V1 should also include a positive DC bias of around 6V for the transistor circuit to 
work. Otherwise you could just be getting half wave rectified pulses coming out on the positive peaks of 
the AC input waveform.

The opamp example probably assumes the opamp is powered by bipolar supply rails so it works with no 
DC bias on the input.


On Mon, Nov 19th, 2018 at 10:59 AM, Tom Wiltshire <tom at electricdruid.net> wrote:

> Can anyone tell me why a transistor-based Sallen-Key filter like this one
> would have such a heavy roll-off at the bass end?
> 	https://electricdruid.net/wp-content/uploads/2018/11/DelayFilterSchematic.png
> According to the sim (LTSpice), the response looks like this:
> 	https://electricdruid.net/wp-content/uploads/2018/11/DelayFilter.png
> There’s a significant drop with a -3dB cutoff of about 200Hz. What’s
> causing that?
> Using the same passive components, but with a op-amp in place of the tranny,
> with the tools at..
> 	http://sim.okawa-denshi.jp/en/Sallen3tool.php
> ..doesn’t give the same effect. Am I seeing a simulation effect or a real
> phenomena?
> Thanks,
> Tom
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